Spark plugs having a plurality of electrodes for providing multiple sparks during each firing cycle of the plug are well known.
Hilliker et al U.S. Pat. No. 1,125,097, Jan. 19, 1915, discloses a spark plug having a pair of fixed electrodes spaced to either side of a rotatably mounted neutral electrode. The neutral electrode can be set with respect to the fixed electrodes to provide either a single or a double spark gap.
Carpenter U.S. Pat. No. 1,505,373, Aug. 19, 1924, discloses a spark plug having a rotatably mounted center electrode and a plurality of ground electrodes, the center electrode having a multi-pointed head. Interposed between the center and ground electrodes are a plurality of neutral electrodes. Upon firing, a double spark occurs between the center electrode, a neutral electrode and a ground electrode.
Thompson U.S. Pat. No. 1,660,635, Feb. 28, 1928, discloses a spark plug having two spark gaps. One gap is formed between a ground electrode and the peripheral surface of a flat metal plate and the other gap is formed between a center electrode and the wall defining a hole formed in the plate.
Stahr U.S. Pat. No. 2,208,059, July 16, 1940, discloses a spark plug in which the main electrode is mounted off-center with respect to the axis of the core. A neutral electrode extends from an end of the core and is disposed between the main electrode and a ground electrode.
The prior constructions are expensive and difficult to manufacture. Spark plugs made in accordance with this invention are of simple, rugged and inexpensive construction, and are readily adapted to provide either two or more than two sparks during each firing cycle.